At present, the search for hidden weapons and other smuggled articles is typically carried out by using metal detectors and physical frisking performed on a person. These methods have several downsides. Metal detectors are prone to produce false alarms, nor do they detect non-metallic objects, such as explosives and drugs. On the other hand, a physical frisking performed by the customs officer is tedious and slow. X-rays or other ionizing radiation cannot be used for the inspection of persons because of the health hazards caused by ionizing radiation. Indeed, there is a rapidly increasing demand for a non-contacting inspection method which is harmless for the subject and the inspector.
The non-contacting inspection of a subject is most often effected by using electromagnetic radiation or particle radiation. The interaction between radiation and tissue must be as insignificant as possible in order to provide an inspection method which is as safe as possible. In practice, tis requirement precludes the use of ionizing radiation, for example X-radiation, neutrons, or charged particles. The scattering of very low-energy X-radiation is used to some extend, but the health hazard associated even with that and the mere through of applying ionizing radiation for the inspection of people preclude the use of such systems. The equipment based on the application of ultrasound is completely safe, but the picture quality and resolution achieved thereby are not sufficiently good.
One highly promising procedure for the inspection of people and things is the use of submillimeter-range radiation. This range of the electromagnetic spectrum lies within the intermediate zone of long-wave infrared radiation and microwave radiation, wherein the radiation typically has a wavelength in the order of 100-1000 .mu.m, a frequency of 0,3-3 THz, and a photon energy of 1,2-12 meV. Over this spectral area, most dielectric materials, such as clothes, paper, plastics, and leather, are almost transparent. By virtue of a relatively small wavelength, the diffraction hardly restricts the obtainable resolution, even with relatively small-aperture optics. One of the best features of this wavelength range is that he inspection can be carried out by using a totally inactive system, the examined object being the source of perceivable radiation.
All bodies emit electromagnetic radiation. The radiation emitted by bodies or things complies with a continuous spectrum is known as the black body radiation. The intensity of radiation depends on the temperature of a body and also very critically on how effectively the body radiates. This effectiveness, emissivity is close to one with water-containing material, such as skin. On the other hand; metals have an emissivity which is very close to zero. All other materials lie between these extremes and can be observed according to the radiometric temperature thereof. Metallic and many plastic bodies can be made to appear radiometrically "hot" or "cold" regardless of the bodies' own temperature, as such bodies emit very little radiation and reflect most of the thermal radiation falling thereon, which in turn can be "hot" or "cold". Water-containing materials emit very effectively and, respectively, reflect very little, whereby such materials at body temperature appear to be radiometrically "hot" when compared to bodies at room temperature. Such intensity differences can be measured with a sensitive detector, and a detector matrix can be used for producing an image of the measured object.
There are several different types of detectors functioning over a submillimeter range. The frequency range of radio technology based heterodyne and direct-detection receivers is restricted below 200 GHz with current technology. In addition to these, there are available several different types of bolometers and Golay's cells and pyroelectric detectors. Antenna coupled bolometers are highly suitable for imaging applications, as they have a good signal-to-noise ratio, they are fast by virtue of their low time constant and their manufacturing costs are low. The wavelength range for antenna coupled bolometers can be chosen quite freely, since the sensitivity over various wavelengths is essentially only defined by the design of the antenna.
The know technology functions typically over the range of 80 GHz to 140 GHz, since those frequencies enable the use of traditional microwave technology. This type of solutions are described e.g. in the article P. F. Goldsmith, C.-T. Hsieh, G. R. Huguenin, J. Kapitzky, E. L. Moore, "Focal Plane Imaging Systems for Millimeter Wavelengths", IEEE transactions on microwave theory and techniques, vol. 41, no. 10, Oct. 1993, pp. 1664-1675, as well as in Patent publications U.S. Pat. No. 5,047,783 and U.S. Pat. No. 5,227,800. The article G. R. Hugenin, C.-T. Hsieh, J. Kapitzky, E. L. Moore, K. D. Stephan, A. S. Vickery, "Contraband detection through clothing by means of millimeter-wave imaging", Proceedings of Underground and Obscured Object Imaging and Detection, 15-16 Apr. 1994, Orland, Fla. SPIE proceedings series vol. 1942, pp. 117-128, discloses a system which uses scanning image formation and functions over the 94 GHz frequency range. The traditional microwave technology has the downside that detectors are expensive and, thus, the manufacturing costs of a large detector matrix become very high. In addition, such detectors have a high power consumption. Another drawback with this frequency range is the fact that, as a result of a relatively long wavelength, the diffraction limits the resolution to a modest level.
FIG. 1 depicts a basic structure for an antenna coupled bolometer 10. The antenna coupled bolometer 10 consists typically of an antenna element, including two antenna branches 12 for receiving electromagnetic radiation, as well as of a bolometer element 11 for converting the received electromagnetic radiation into heat. The temperature change of the bolometer element 11 produced by the energy of electromagnetic radiation is detected as a change in the resistance of the bolometer element 11. The bolometer element may also consist of more than one component, whereby the thermalization resistance converting the energy of electromagnetic radiation into thermal energy and the temperature gauge of the thermalization resistance are separate components.
The antenna coupled bolometer is in principle broad-band, but the bandwidth can be affected by the design of the branches 12 of the antenna element. There are a variety of prior known antenna element solutions, the design shown in FIG. 1 being but one example. In addition to the spiral design depicted in FIG. 1, prior known are e.g. a logperiodic design as well as an angular, dipole and double dipole design. Several of these antenna types function well also as complementary designs. Such complementary designs include e.g. a slot antenna, which is a complementary form to the dipole antenna, and a double slot antenna, which is a complementary form to the double dipole antenna. The selection of an antenna design can be used for affecting the characteristics of an antenna, and thereby those of an antenna coupled bolometer, such as for example the frequency band and directional pattern.
Various bolometer solutions, e.g. materials used therein, are described in the review type of article P. L. Richards, "Bolometers for infrared and millimeter waves", Journal of Applied Physics, 76 (1994) 1-24. Various other types of antenna solutions designed by using manufacturing techniques of integrated circuits are described for example in the article G. M. Rebeiz, "Millimeter-Wave and Terahertz Integrated Circuit Antennas", Proceedings of the IEEE, Vol. 80, No. 11, November 1992, pp. 1748-1769.